I came across a published paper that provides the following formula for estimating the specific conductivity (σ) of a metal coating on fibers:

σ=L/π⋅d⋅t⋅R​

where ddd is the average diameter of the fiber (25.1 μm), ttt is the thickness of the coating (550 nm), L is the fiber length, and R is some resistance value. Is this formula correct for calculating the specific conductivity of the coating, especially given that it uses the fiber's average diameter and coating thickness in this form? Are there any important assumptions or limitations in applying this equation that I should be aware of?